1. Technical Field
The invention relates to a liquid crystal display element and a light-modulating material. In particular, the invention belongs to technical fields of a guest-host liquid crystal display element and a light-modulating material.
2. Description of the Related Art
With the spread of the digital technology, the importance of a display for displaying digital information (hereinafter referred to as “electronic paper”) has been increasing. The performance required for the electronic paper includes a high visual recognition and low electric power consumption. High visual recognition means white background similar to paper, and hence a display method based on light-scattering white background similar to paper is suited. On the other hand, as to the electric power consumption, the reflection type display system has lower power consumption as compared with that of self light-emission display system. Many systems have been proposed so far for the electronic paper. Examples include a reflection type liquid crystal display system, electrophoresis display system, magnetophoresis display system, dichroic ball rotation system, electrochromic display system, and leucothermal system. Any of these methods is not satisfactory from the viewpoint of high visual recognition, and improvement therefore has been demanded.
The importance of light-modulating materials as environment-friendly materials has increased. Regarding the light-modulating materials, while light-modulating methods that have been investigated include, in addition to light and heat, modulation by stimulating with electrical modulation methods, but these methods are not yet at a satisfactory level, and further improvements are required.
While many display methods have been proposed for the liquid crystal display element, among the various methods a guest-host liquid crystal display element looks promising since bright display is possible with such an element and it is applicable to reflection display methods. A guest-host liquid crystal display element displays by placing a liquid crystal composition including a dichroic dye dissolved in a nematic liquid crystal into a cell and sealing it, applying an electric field, and changing the orientation of the dichroic dye in accordance with the movement of the liquid crystal by the electric field thereby changing the light absorption state of the cell. In guest-host liquid crystal display elements, different from in conventional liquid crystal display methods, since a driving method requiring no polarizer is possible, brighter displays may be expected. Examples of proposed guest-host methods using no polarizer, that is to say methods for realizing a light absorption state that are not dependent on polarized light, include a method taking advantage of a chiral nematic phase by a combination of a liquid crystal and a chiral reagent, and a polymer dispersion method (PDLC) taking advantage of complexing a liquid crystal with a polymer matrix. However, display performance of these methods is not at a satisfactory level, and improvements of the methods are required.
A liquid crystal composition to which a low molecular weight gelling agent is added has been disclosed for a liquid crystal display element comprising a nematic liquid crystal (for example, see J. Mater, Chem. Vol. 12, p 2197-2201, 2002). The patent publication describes that switching between scattering/transmission states is possible by gelling the nematic liquid crystal.
While a host-guest method using the low molecular weight gelling agent has been disclosed (for example, see Japanese Patent Application Laid-Open (JP-A) No. 2001-21924), display performance thereof is not at a satisfactory level yet.